Received 1999 June 16; accepted 1999 July 23; published 1999 August 25

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1 The Astrophysical Journal, 3:L11 L1, 1999 September The American Astronomical Society. All rights reserved. Printed in U.S.A. X-RAY SPECTRAL VARIABILITY OF PKS DURING THE SPECTACULAR 1998 NOVEMBER FLARE Eric S. Perlman, 1 Greg Madejski, John T. Stocke, 3 and Travis A. Rector 4 Received 1999 June 16; accepted 1999 July 3; published 1999 August ABSTRACT We report on monitoring of the BL Lac object PKS by the Rossi X-Ray Timing Explorer (RXTE) in 1998 October December. During these months, the source underwent a spectacular flare; at its peak on 10 November 10, its 10 kev flux was 3.33 # 10 ergs cm s 1, over 30 times brighter than in quiescence. During the rising phase, the X-ray spectrum of PKS hardened considerably, reaching a = a 1.3 (Fn n ) near maximum. During the declining phase, the X-ray spectrum steepened rapidly, reaching a = 1.8, then became somewhat harder toward the end of December ( a 1.6). While such behavior has been seen before, the simplicity, magnitude, and duration of this flare allowed us to study it in great detail. We argue that this flare was caused by either the injection of particles into the jet or in situ particle acceleration and that the spectral steepening which followed the flare maximum was the result of synchrotron cooling. Contrary to other recently observed blazar flares (e.g., Mrk 01, 3C 79, PKS 1 304), our results do not imply a major shift in the location of the synchrotron peak during this flare. Subject headings: BL Lacertae objects: individual (PKS ) galaxies: active X-rays: galaxies 1. INTRODUCTION BL Lacertae objects are characterized by highly variable nonthermal emission, which dominates their characteristics from the radio through the g-rays. The mechanism believed to be responsible for their broadband emission is synchrotron radiation followed by inverse-compton scattering at higher energies (e.g., Blandford & Königl 1979). Relativistic beaming of a jet viewed at very small angles is the most natural explanation for the extreme properties of the class, which include violent variability (up to 1 mag in the optical; see Wagner & Witzel 199 and references therein), high g-ray luminosities (Mukherjee et al. 1997), featureless optical spectrum, and superluminal motion (Vermeulen & Cohen 1994 and references therein). The BL Lac class is often separated into two subclasses via the location of the peak in the synchrotron part of their spectrum. High-energy peaked BL Lac objects (HBLs) have synchrotron peaks in the UV/X-ray, while low-energy peaked BL Lac objects (LBLs) peak at lower energies, in the IR/optical. The reality and/or nature of this apparent dichotomy is a subject of active debate in the literature (e.g., Urry & Padovani 199; Padovani & Giommi 199; Georganopoulos & Marscher 1998a). PKS is a bright, z=0.071 BL Lac object, with a broadband spectrum that peaks in the UV, making it an HBLtype or intermediate object (Sambruna et al. 199; Perlman et al. 1996). As the only confirmed (few) extragalactic TeV g- ray sources are nearby HBLs (Catanese & Weekes 1999 and references therein), PKS has recently become a prime target for southern-hemisphere Cherenkov observatories. Here we report on the X-ray spectral variability of PKS during its 1998 November flare. We will report on multiwaveband observations in a future paper (Perlman et al. 1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 118. Laboratory for High Energy Astrophysics, NASA/Goddard Space Flight Center, Greenbelt, MD 0771; also with the Department of Astronomy, University of Maryland, College Park. 3 Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO National Optical Astronomy Observatories, 90 North Cherry Avenue, Tucson, AZ L b). All calculations assume q 0 = 0.1. H = OBSERVATIONS km s 1 Mpc 1 and RXTE observations of PKS were organized around multiwavelength campaigns in 1998 September and October. The October campaign began October 14 1, intending to observe the object for 10 ks per day for days. Due to hardware problems, observations were suspended October 16 0 and October 3-9. During the week of October 18-, a flare alert was issued by the all-sky monitor (ASM) team (Remillard 1998); following consultations with the current authors and the ASM team, monitoring continued through the end of 1998, with observations of 1 10 ks duration every 3 days. Two instruments aboard RXTE are designed to observe the X-ray spectrum and variability of target sources. The PCA (Jahoda et al. 1996) consists of five co-aligned, gas-filled proportional counter X-ray detectors (called PCUs), sensitive between 60 kev, each of which has an open area of 1300 cm. The PCA has a roughly triangular response over its nearly 1 field of view. During all but one observation three or four PCUs were in use. For maximum consistency, we use PCUs 0, 1, and in our data analysis. The HEXTE (Rothschild et al. 1998) consists of two clusters, each having four NaI/CsI scintillation counters, sensitive over the 1 0 kev range, with a total effective area of 1600 cm, and a 1 field of view. The source was detected up to 40 kev with HEXTE, but significant signal was obtained only when all HEXTE data were combined. For this reason, we will use only the much higher signal-to-noise PCA data in the ensuing discussion. Light curves were extracted using the ftools SAEXTRCT and LCURVE. The variable PCA background was modeled with PCABACKEST, which uses observations of X-ray blank, high latitude areas of sky (Jahoda et al. 1996). X-ray spectra were extracted using the script REX. Spectral fits were done in XSPEC v. 10.0, using all three xenon levels and response matrices generated by the ftool PCARMF. Fits were done over The exception is November 6, when PCUs 1 and were in use.

2 L1 X-RAY SPECTRAL VARIABILITY OF PKS Vol. 3 the ranges kev (PCA layer 1) and kev (layers and 3), ignoring channels around the 4.8 kev Xe feature. Morrison & McCammon (1983) cross sections, solar 0 abundance and N(H) = # 10 cm were assumed, consistent with BeppoSAX and ROSAT observations (Padovani et al. 1999; Sambruna et al. 199). 3. FLUX AND SPECTRAL VARIABILITY During the last quarter of 1998, PKS underwent a spectacular X-ray flare, comparable to the largest observed in any BL Lac object. Variability of a factor 8 was detected (Fig. 1, top three panels; the variations are larger in the harder bands), and the source peaked at F( 10 kev) = 3.33 # ergs cm s 1 on 10 November. A fuller appreciation of the magnitude of the flare is obtained by comparing with two previous ROSAT observations (extrapolated assuming a = 1.9; Sambruna et al. 199) and two EXOSAT observations (Sambruna et al. 1994), which show that a more average 10 1 kev flux for PKS is 9 # 10 ergs cm s Somewhat higher fluxes respectively, 3. and 6.1 # 10 ergs cm s 1 were recorded during 1997 observations by RXTE and BeppoSAX (Lamer et al. 1999; Padovani et al. 1999). Thus, at its peak in 1998, PKS was about 3 times as bright as in quiescence, with a 10 kev luminosity (assuming isotropic emission) of 4.8 # 10 4 ergs s 1. By comparison, the great 1997 flare of Mrk 01 (Pian et al. 1998) represented an increase in 10 kev flux of slightly less than a factor of 0 (peak to quiescence; N.B., during its flare Mrk 01 s synchrotron spectrum peaked at 100 kev) and a peak 4 luminosity of 1.9 # 10 ergs s 1. These observations show that PKS was in an enhanced flux state for at least 3 months. The flare s peak is characterized by a plateau lasting about 4 days (Fig. 1, top three panels), and the half-maximum region spans 3 days. No variability is found on timescales of less than 1 day; however, our sampling on any given day covers at most 10 ks. Both the rising and declining phases show plateaus lasting 1 weeks near half maximum flux. The most striking feature one obtains from Figure 1 is that the emission in the harder bands declines faster. Due to the lack of sharp features in the light curve and the somewhat sparse sampling, this is difficult to quantify via cross-correlations; indeed, by cross-correlating the kev and 10 kev light curves, we find no evidence of a lag. To crudely characterize the light curve, we take the rise time (defined as the increase of flux from 0% to its peak value) to be 9 # 10 s. For the decline, the drop of the flux to 0% of peak for 3, 7, and 1 kev photons (corresponding to the weighted mean energy of the photons in the, 10, and 10 1 kev bands) is, respectively, 18, 14, and 11 # 10 s. A power law plus Galactic absorption was an excellent fit to the data on all but two days, October 14 and 1. In those cases, we fit a broken power-law model to the data. The spectral fits are shown in Table 1 (single power-law models) and Table (broken power law). An F-test shows that the broken powerlaw models cited in Table are a better representation of both the October 14 and 1 data, at greater than 99.9% significance. Given the more rapid drop observed in the hard X-rays, it is not surprising that the X-ray spectrum showed significant evolution during the flare, following a loop in the spectral index-flux plane which has been seen in several other BL Lac flares (see 4 for discussion). In Figures 1 (bottom) and, we Fig. 1. Here we show the RXTE PCA count rates and energy spectral indices for PKS during 1998 October December. Count rates are given in the kev (top), 10 kev (second from top) and 10 1 kev bands (third from top), while the spectral indices we plot (bottom) are for the 10 kev band. Given the observed spectra, the weighted mean energy of the photons in the kev band is 3 kev, in the 10 kev band it is 7 kev, and in the 10 1 kev band it is 1 kev. These values are spectral index dependent, but change only a few percent for the spectra observed. On this figure, a time of 0 s refers to the beginning of the first (October 14) observation, and the peak, which occurred on November 10, is at.4 # 10 6 s. show (respectively) the progression of the X-ray spectral index with time and flux. When RXTE observations commenced October 14, the X-ray spectrum of PKS was already flatter than in quiescence ( a = 1.4 compared to a = ), and in fact was slightly concave, hardening by Da = 0.3 above 9 kev. The spectrum hardened slightly as PKS continued to brighten, reaching a = 1.3 on November 6, 4 days before the peak. The slope remained roughly constant during the flux maximum, and then steepened precipitously as the flare began to decline; exactly when this change begins is difficult to pinpoint due to the lack of observations during November The steepest spectrum was observed on December 11 ( a = 1.8), after which the spectrum hardened once again, as the flux continued to decrease. 4. DISCUSSION Recent observations of BL Lac objects have found that flares seem to be accompanied by a general hardening of the spectrum: not only does the flux increase at every frequency, but

3 No. 1, 1999 PERLMAN ET AL. L13 TABLE 1 Single Power-Law Spectral Fits Date (1998) F ( 10 kev) (#10 11 ergs cm s 1 ) a a (F n ) n x n (133 channels) Oct a 1.47 Oct Oct Oct Oct Oct Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Nov Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec Dec a All errors given are 90% confidence. the peak of the synchrotron emission (and presumably higher energy inverse-compton emission) moves further to the blue, sometimes by large factors (e.g., #100 during a large flare of Mrk 01, Pian et al. 1998; and #30 during a somewhat smaller flare of 1ES ; Giommi, Padovani, & Perlman 1999a). How this manifests in the X-ray band depends on the shape of each source s broadband spectrum. The most dramatic behavior (Da up to 0.6) is seen in HBLs, which often follow a looping pattern in the spectral index-flux plane. The X-ray spectrum hardens prior to the peak, steepens in the declining phase, and then hardens again as the flux continues to decrease. This pattern has been observed in several HBLs, including Mrk 41 (Takahashi et al. 1996; Takahashi, Madejski & Kubo 1999), PKS (Sembay et al. 1993; Chiappetti et al. 1999) and Mrk 01 (Pian et al. 1998). There are, however, exceptions: spectral steepening with increasing flux has been seen twice in PKS (Sembay et al. 1993). Different behavior is observed in objects that peak at lower energies. In LBLs, the X-ray spectrum is probably dominated by inverse-compton radiation (Sambruna, Maraschi, & Urry Fig.. Progression of spectral index with flux during the 1998 flare. For comparison, we show ROSAT and EXOSAT observations (Sambruna et al. 1994, 199) as triangles, and previous RXTE and BeppoSAX observations (Lamer et al. 1999; Padovani et al. 1999) as diamonds. All error bars are 90% confidence. The arrows indicate the general time sequence. 1996; Padovani, Giommi, & Fiore 1997; Kubo et al. 1998), and so only modest changes tend to be observed, though some hardening was seen during a recent flare of BL Lac (Madejski et al. 1999). However, objects with peaks between and 10 1 Hz behave very differently. S (Giommi et al. 1999b) and AO (Madejski et al. 1996) exhibited X-ray spectral steepening in flare states (see also Urry et al. 1996), probably because the X-ray spectrum was dominated by inverse-compton emission in the low state, but by the soft tail of the synchrotron emission in the high state. During this flare, the X-ray spectrum of PKS responded similarly to other HBLs, but with larger changes in the declining phase (where a = , similar to values ob- tained in quiescence) than in the rising phase (Figs. 1 [bottom] and ). Importantly, our data do not imply a large shift in the location of the synchrotron peak. This can be seen by contrasting a low-state peak at Hz ( 0.1 kev; Sambruna et al. 199; Padovani et al. 1997) with X-ray spectral indices uniformly steeper than 1, meaning that the synchrotron peak cannot have moved to Hz, i.e., a factor of 10. This flare of PKS thus appears to have been more similar to flares of Mrk 41 (Takahashi et al. 1999) than of Mrk 01 (Pian et al. 1998) or 1ES (Giommi et al. 1999a), where much larger shifts in the spectral peak were observed. This could indicate a diversity in physical circumstances in blazar flares such as the balance between acceleration, particle injection and cooling. The usual interpretation of the X-ray spectral changes observed in HBLs (see models by Georganopoulos & Marscher 1998b; Kirk, Mastichiadis, & Rieger 1998; and Coppi & Aharonian 1999) is that the hardening observed during the flare s rising phase is caused by reacceleration and/or injection of electrons (either freshly injected or already within the jet). This Date (1998) Oct Oct TABLE Broken Power-Law Spectral Fits F ( 10 kev) 11 1 (#10 ergs cm s ) a soft a hard E break x n (133 channels)

4 L14 X-RAY SPECTRAL VARIABILITY OF PKS Vol. 3 may be associated with a local compression or augmentation of the jet magnetic field (as has recently been observed in HST polarimetry of knots of the M87 jet which have superluminal components; see Perlman et al. 1999a). In the declining phase, synchrotron or Compton cooling is the dominant factor. We therefore interpret the flare as follows: the increase of the flux is due to a fresh injection or acceleration of energetic particles throughout the source, which we assume happened on a timescale that is (locally) much shorter than the light travel time across the source. In this context, the timescale of the increase of the flux #c may well be an indication of the source size. The presence of a somewhat harder component in the spectra of October 14 and 1 indicates considerable structure in the flaring region which these data could not resolve. This component may be associated with the onset of either a second, high-energy synchrotron component, or the onset of Comptonization. The decrease of the flux which is slower than the rise for all energies is caused by those particles cooling via synchrotron radiation. No g-ray emission has so far been detected for PKS , so the synchrotron component is most likely more luminous than the Compton component. We therefore assume that the synchrotron losses dominate. As we mentioned above, it is now widely accepted that the broadband emission in blazars arises in a relativistic jet pointing close to the line of sight. This results in observed timescales being shortened, and the detected photons being blueshifted by 1 a factor d = [G # (1 b cos v)], where G is the bulk Lorentz factor of the jet, b = v/c, and v is the jet viewing angle with respect to the observer. Furthermore, since the emission is not isotropic (and relativistically boosted in the direction of motion of the jet), the luminosity inferred under an assumption of isotropy is an overestimate. We have no information regarding d for PKS , but in the ensuing discussion, we take d = 10, which is the value inferred for many other blazars (e.g., Ulrich, Maraschi, & Urry 1997). For a homogeneous emitting region, the radiative lifetime of a relativistic electron emitting synchrotron photons with energy E kev is (in the observer s frame) t sync = 1. # 3 3/ 1/ 1/ 10 B E kev d s (cf. Rybicki & Lightman 1979). This should give us at least some estimate of the magnetic field B. Of course we do not know the extent to which the timescale of the flux decrease was due to the propagation of the signal throughout the source and to what extent it was caused by the synchrotron cooling. However, our data allow us to measure the drop of the flux in three energy bands, where t 1/(E) = 18, 14, and 11 # 10 s, respectively, for 3, 7, and 1 kev photons (cf. Fig. 1). Following Figure 4 of Takahashi et al. (1996), we can write 3 3/ 1/ 1/ 1/ t sync(e) t sync(1 kev) = 1. # 10 B d (E kev 1 ). Comparing the decline of the 7 kev and 1 kev photons, we 1/3 infer B = (d/) G; comparing the decline of the 3 kev versus 1 kev photons, we obtain B = (d/) 1/3 G. With 3 B = 4 # 10 G, the Lorentz factors of the electrons g el radiating at energy E can be estimated from E = 4 # 1 10 g el(d/) kev. This implies that g el of electrons radiating in the X-ray band is in excess of (Note that g el refers to the Lorentz factors of individual radiating particles as distinct from G.) The value of B calculated as above is significantly lower than 0. G, and g el higher than # 10 inferred for Mrk 41 by Takahashi et al. (1996). This may be either due to a real difference between the two sources (note that the inferred isotropic luminosity of PKS is a factor of a few greater than that of Mrk 41), or because t sync inferred by us is an underestimate. This last possibility is likely, if the flare observed by the RXTE consists of a superposition of multiple, rapid variability (shorter than 1 day) events unresolved by this observation. An absence of such short-timescale variability would be rare for BL Lac objects (Wagner & Witzel 199). If in fact variability on timescales shorter than a day is present, the value of B we calculate is instead a lower limit, and g el is an upper limit. A conclusive test of the physical parameters in this source would be a detection of the TeV g-ray emission, and any correlation with the X-ray flux. These uncertainties underline the importance of future campaigns and modeling efforts. Better, more dense sampling might tell us if a large flare actually consists of many smaller but more rapid flares. The differences between the behavior of PKS during this flare, and that of other flaring blazars, point out that as yet we do not understand well how blazar jets respond to stimuli. Such models will need to include a large number of factors, meaning not only the injection and reacceleration of particles, but also compression of magnetic fields in shock regions (which both accelerates electrons and shortens their dynamical lifetimes). Several authors have begun to build models which take these effects into account (e.g., Kirk et al. 1998, Georganopoulos & Marscher 1998b); however, intensive multiwavelength monitoring of many more flares (in both HBL and LBL type objects) will be needed to reach a complete understanding. We wish to thank the RXTE ASM team at MIT, and the RXTE team at GSFC, for their rapid response to the flare of PKS during October 1998 and for continuing observations through December 31. We wish to thank the staff of the RXTE GOF, and particularly Tess Jaffe, for their help in reducing the data from these observations. E. S. P. acknowledges support from NASA grant NAG-71. We wish to thank our referee, Rita Sambruna, for many helpful comments which significantly improved this Letter. REFERENCES Blandford, R., & Königl, A. 1979, ApJ, 3, 34 Catanese, M., & Weekes, T. 1999, PASP, in press (astro-ph/990601) Chiappetti, L., et al. 1999, ApJ, 1, Coppi, P., & Aharonian, F. 1999, ApJL, submitted (astro-ph/990319) Georganopoulos, M., & Marscher, A. 1998a, ApJ, 06, b, ApJ, 06, L11 Giommi, P., Padovani, P., & Perlman, E. 1999a, MNRAS, in press (astro-ph/ ) Giommi, P., et al. 1999b, A&A, submitted Jahoda, K., Swank, J. H., Giles, A. B., Stark, M. J., Strohmayer, T., Zhang, W., & Morgan, E. H. 1996, in Proc. SPIE, 808, 9 Kirk, J. G., Rieger, F. M., & Mastichiadis, A. 1998, A&A, 333, 4 Kubo, H., Takahashi, T., Madejski, G., Tashiro, M., Makino, F., Inoue, S., & Takahara, F. 1998, ApJ, 04, 693 Lamer, G., et al. 1999, in preparation Madejski, G. M., Sikora, M., Jaffe, T., Blazejowski, M., Jahoda, K., & Moderski, R. 1999, ApJ, 1, 14 Madejski, G. M., Takahashi, T., Tashiro, M., Kubo, H., Hartman, R., Kallman, T., & Sikora, M. 1996, ApJ, 49, 16 Morrison, R., & McCammon, D. 1983, ApJ, 70, 19 Mukherjee, R., et al. 1997, ApJ, 490, 116 Padovani, P., & Giommi, P. 199, ApJ, 444, 67 Padovani, P., Giommi, P., & Fiore, F. 1997, MNRAS, 84, 69 Padovani, P., et al. 1999, in preparation

5 No. 1, 1999 PERLMAN ET AL. L1 Perlman, E., Biretta, J., Zhou, F., Sparks, W., & Macchetto, F. 1999a, AJ, 117, 18 Perlman, E. S., Stocke, J. T., Wang, Q. D., & Morris, S. L. 1996, ApJ, 46, 41 Perlman, E. S., et al. 1999b, in preparation Pian, E., et al. 1998, ApJ, 49, L17 Remillard, R. 1998, IAU Circ Rothschild, R., et al. 1998, ApJ, 496, 38 Rybicki, G. B., & Lightman, A. P. 1979, Radiative Proceses in Astrophysics (New York: Wiley) Sambruna, R. M., Barr, P., Giommi, P., Maraschi, L., Tagliaferri, G., & Treves, A. 1994, ApJ, 434, 468 Sambruna, R. M., Maraschi, L., & Urry, C. M. 1996, ApJ, 463, 444 Sambruna, R. M., Urry, C. M., Ghisellini, G., & Maraschi, L. 199, ApJ, 449, 67 Sembay, S., et al. 1993, ApJ, 404, 11 Takahashi, T., et al. 1996, ApJ, 470, L89 Takahashi, T., Madejski, G., & Kubo, H. 1999, Astropart. Phys., 11, 177 Ulrich, M.-H., Maraschi, L., & Urry, C. M. 1997, ARA&A, 3, 44 Urry, C. M., & Padovani, P. 199, PASP, 107, 830 Urry, C. M., Sambruna, R. M., Worrall, D. M. Kollgaard, R. I., Feigelson, E. D., Perlman, E. S., & Stocke, J. T. 1996, ApJ, 463, 44 Vermeulen, R., & Cohen, M. 1994, ApJ, 430, 467 Wagner, S. J., & Witzel, A. 199, ARA&A, 33, 163

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